Roofing Slate Inspection Checklist: What to Check When the Container Opens

Part of the Roofing Slate Guide's Quality, Testing & Standards section. This is the receiving end of the story: our test-report walk-through decodes the paper, and this page is what you do when the paper and the pallets finally meet at your door.

Here's the moment I keep telling buyers to slow down, and they never do. The truck is in the yard, the driver's got a clipboard, and somewhere behind that steel door are the slates you've waited four months for. You want to tear the crate open. You want to see the colour, touch the surface, start planning the roof. That's exactly the wrong instinct — and it's the most expensive one in the whole slate trade.

Twenty-odd years of inspecting slate — for importers, roofing contractors, even the occasional heritage trust — taught me one sentence that covers the entire job: the inspection happens in the first hour, or it doesn't happen at all. Because the moment those crates hit the forklift and the pallets start moving, the evidence scatters. You can't claim a broken tile that's been sitting in the yard for a week, and you can't photograph a batch mix-up after the crates are stacked three high. So what follows is not a list of things to check someday. It's the exact order to run them in, with the numbers to judge them against, in the first sixty minutes after the door opens.

Before you touch a tile: the paperwork check

Everything else on this page is about the stone. But the stone check only makes sense if the stone matches the paper. So the first ten minutes go to paperwork, and I do it in this exact order:

• 1. The packing slip vs. the purchase order. Size, colour, thickness, quantity, grade — five numbers. If the slip says 500×250 and you ordered 500×250, good. If it says something else, stop and call the supplier before you open anything. This is the cheapest moment to catch a problem.
• 2. The test report and batch number. You read the test report when you bought the stone. Now check the batch number on the report against the batch number on the packing list and on the crates themselves. It should be the same number three times. If it isn't — if the paper describes batch A and the crate says batch B — the report is describing someone else's slate, and that's a conversation to have before the container is unloaded.
• 3. The crate count. The slip says 400 crates, 8 pallets per pack, 5,000 tiles. Count the crates while they're still stacked. It takes two minutes and it's the one check that can't be undone after.

That's the paperwork pass. It's boring, and it's the highest-value five minutes in the whole delivery. Because the second the fork truck starts moving crates, the "which batch was where" information is lost forever.

Then open one crate: the ten-minute visual pass

Not all of them. One crate, from the middle of the stack, not the easiest one on top. Because that's the one that's been handled most and shows it. Cut the straps, pull the lid, and work through the tile checks in this order:

Check	What you're looking at	Fail if…
Colour & batch	Are the tiles the colour you approved? Are there two visibly different batches in the same crate?	Two tones in one crate, or the colour doesn't match your approved sample.
Size	Take a tile and measure length, width and thickness with a calliper.	Out of tolerance (see the thickness section below — ±0.5 mm is realistic for CNC work, ±1 mm or more for hand-split).
Edges	Run a thumbnail along each edge. Look for chips, crumbling, or edge lift.	Chipped edges on more than 2–3% of the tiles, or any edge that crumbles under a fingernail.
Ring test	Hold a tile by the corner and tap it lightly with a steel trowel or hammer handle.	A dull thud instead of a clean ring — the stone is delaminated, or hiding an internal crack.
Surface	Look for pyrite dots, carbonate ribbons, rust streaks, or a waxy surface.	Visible pyrite that catches light, soft carbonate bands, or a sheen that says the surface was waxed to hide flaws.

The ring test is the one the roofers all know and nobody writes down, so say it twice: clean slate rings like a bell. A cracked or delaminating tile gives a dull, dead thud. You don't need perfect pitch — you need the difference between "ring" and "thud". Every tile in your sample crate should ring. If a third of them thud, that's not an unlucky crate, that's a bad batch.

The thickness check: where the CNC shows

Thickness is the single dimension that changes everything else about a roof — the weight, the fixing, the headlap, the coverage. And it's the one most sloppy deliveries fail on. Measure at least five tiles from your sample crate, at each of the four edges, and note the spread.

The practical numbers, from a slate that's been CNC-dressed (which is the whole point of how the tile was made):

Slate surface	Realistic thickness spread	Why it matters
CNC-dressed	±0.5 mm	Roof weight is predictable, fixing is even, the wall that was pitched for 6 mm actually gets 6 mm.
Hand-split	±1 mm or more	Characterful, but the roof gets a random mix — more waste, more work.

And the rule that catches more bad loads than any other: write down the thickness you find, then check it against the thickness on the packing slip. If the slip says 6 mm and your calliper says 4.8 mm on the average, that's not a tolerance question, that's a different product. Keep the calliper out — you'll need it for the next check.

The four defects you can't un-see

In twenty years I've developed a short list of defects that are — to borrow a phrase from the roofing trade — "deal-breakers" on inspection. Not every flaw kills a slate; some are cosmetic. These four change the performance of the tile, and you should know their names:

• 1. Delamination — the slate's layers have started to separate. You'll see it as a light-coloured edge or a flaky corner. It means the tile will peel like a book over years. Delamination is a full reject.
• 2. Pyrite spots — small gold or iron-coloured dots that oxidise, swell, and eventually pop small pits out of the face. Some is normal; a face full of them is a waiting façade problem.
• 3. Carbonate ribbons — soft, light-coloured bands that run through the stone. They look pretty and they're a trap: they erode faster than the slate around them, and in a wet climate they'll be the first thing to go. It's the same story we told in the carbonate explainer.
• 4. Edge chipping — a broken corner or a nib that's already gone. A couple of chips are normal in transport. When they're everywhere, your handling is at fault, or the packaging was too rough.

Here's the honest part about breakage: a certain amount of breakage is built into the deal. It moves in a container, it gets strapped, it gets lifted. A responsible importer budgets for it. The word "breakage" is not a four-letter word; the word "unchecked" is. Your job at the gate is to photograph it, count it, and agree with the driver that it's there — then you and the supplier work out whether it's within the agreed allowance or not. That's the entire game. Photograph every cracked tile where it is, from two angles, with a reference to the crate number. If you do nothing else, do this: a picture of the broken tile on the forklift is worth ten emails.

The breakage allowance: a number to talk about

What's realistic? In practice, most reputable suppliers and buyers agree on a breakage allowance in the range of 1–3% of the shipment — that's the crunched corners and snapped tiles that the container's normal motion produces. When you see a single crate that's 15% broken, that's not an allowance, that's a handling failure — either the straps were too tight, the crate was stacked upside down, or the container was dropped. There is a whole section of the buying guide on this, but the inspection version is: the average matters, and the outlier matters more. Photograph the outlier.

Also worth knowing before you open the crate: some suppliers mark the expected breakage on the packing or in their terms. If they don't, the industry default is around 2%. The buyer who asks "what's your allowance?" before signing has already done half the inspection.

The ten-minute close: when to reject, when to accept

At the end of the visual pass, you have enough to make a call. And the call is rarely a black-and-white "reject everything". It's more often a shade of grey, and the professional move is to have a grading rule ready. Here's mine, in full:

Finding	Action
Slip doesn't match order (size, colour, thickness, quantity)	Stop. Don't unload. Email the supplier with the photo before you break anything else.
Ring test fails on >10% of the sample crate	Reject the batch and photograph the evidence. A thud rate of 10% is not bad luck, it's a bad batch.
Thickness spread >±1 mm from the stated spec	Pause. Re-measure 10 tiles before you decide. This is where the calliper saves your roof.
Breakage ≤ 3%	Accept, but photograph everything and sign the packing slip with a note.
Colour is consistent, tiles ring, edges are good	Accept, and stack the crates so the good ones go on the roof first. That's the whole job.

One phrase I want to underline, because every inspector eventually says it to a client: "You can't reject a broken tile after it's been unloaded." The inspection is the only moment the evidence is clean. Walk away from it and you've bought the problem — not the slate, the problem.

The 1-page checklist you can print

Here's the whole thing, condensed to one sheet, in the order to run it. Copy this into your delivery-day folder, screenshot it to your phone, or pin it to the site office wall:

Frequently asked questions

Related Reading

• How to Evaluate Roofing Slate Quality — the factory-floor version of this checklist, for the day you choose a supplier instead of a delivery.
• How to Read a Roofing Slate Test Report — the six rows that decide whether the paper in your hand describes the stone in the crate.
• Roofing Slate Buying Guide & Checklist — the full sourcing-to-import routine that ends at this inspection.
• Roofing Slate Sizes & Standard Dimensions — the size table to check your delivery against.
• Roofing Slate Terminology — every word an inspector throws at you, defined.